1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to an alkali metal electrochemical cell having a positive electrode comprising a cathode active admixture or formulation of a major portion of fluorinated carbon mixed with a minor portion of a metal-containing material. The metal-containing constituent is particularly useful as an end-of-service or end-of-life indicator for the cell.
2. Prior Art
It is known to provide composite cathodes comprising fluorinated carbon for the purpose of providing the cell with an end-of-service indicator. U.S. Pat. No. 5,180,642 to Weiss et al. discloses electrochemical cells having a cathode mixture comprised of manganese dioxide (MnO.sub.2), carbon monofluoride (CF.sub.x) or mixtures of the two and an end-of-service additive selected from the group consisting of vanadium oxide, silver vanadate, bismuth fluoride and titanium sulfide. U.S. Pat. No. 4,259,415 to Tamura et al. provides a positive active mixture as an end-of-service indicator comprising a main positive active material and a precursor. Suitable main positive active materials include molybdenum oxide (MoO.sub.3), silver oxide (Ag.sub.2 O) and graphite fluoride (CF).sub.n while suitable precursor materials are oxyacid salts.
What is needed is a power source that is suitable for powering an implantable medical device wherein the battery includes a mixed cathode formulation of at least two active constituents which are characterized by two, discretely different operating voltages, the second of which may be used as an end-of-life indicator. Further, the provision of an end-of-life indicator must not be provided at the expense of safety especially under short circuit conditions. Compromise in this aspect can have fatal consequences.